Glad to see the ACP getting some discussion. I'll share a few of my thoughts, some of which echo what Elsensee has already written in the OP. I don't see anything in the OP as it is written now that I disagree with.

Some of the confusion about the ACP comes from multiple ways to access the same function. For example, "Manage Forums" is under Quick Access on the General tab, which leads to the same place as the Forums tab. We can reduce clutter by eliminating this duplication.

In addition to multiple pathways to reach the same page, some settings are duplicated. For instance, under Board Features, a number of the Load Settings are duplicated.

Many of the items on the General tab are rarely if ever accessed, and might benefit from being buried deeper, perhaps an interstitial page with better/longer descriptions. For example: Signature settings, feed settings, and avatar settings are probably one-time settings for most admins.

Many settings could be deprecated/removed altogether. Why would an admin want to prohibit users from subscribing to topics!?! Or disable dotted topics? Or database-based unread tracking? Some of these may have made sense in 2006, but they don't seem reasonable today.

The forum management UI is cumbersome to me. It is one of the things I frequent as an admin, splitting up forums from time to time. And it never ceases to confuse me when I get to the end of a chain of subforums and am greeted with a neary-blank page that says: Ludwig van Beethoven [Edit | Delete | Resynchronise]. It seems like this page should say something. Maybe take me to the edit page by default. Maybe display a permissions summary for the selected forum (which is currently pretty cumbersome to get to, itself). A better way to re-arrange forums would be awesome.

Some of the language strings are less than ideal. e.g. Attachment Settings->Maximum file size messaging:. Some (e.g. on the Forum management tab) are trying to explain how the phpBB3 paradigm is different than the phpBB2 one. Well, it's been 10+ years since the phpBB2 paradigm stopped mattering, and we should not assume any modern user ever used phpBB2.

Board Features->"enable quick reply" option should go away. The Internet has quick reply now, turning it off is dumb. (I have the sudden feeling SHS` is going to summon BertieZilla to slap me)

The ACP is not unusable because the number of settings available, but because of their organisation (or lack there of). Eg. "Board settings" and "Board preferences" are pretty *beep* names as you don't really know what you would find there.

It's both. If you want to toggle a particular feature (say the "board disabled" toggle), not only do you have to know/guess which category it is filed under, but then you have to read through a listing of 20 settings on that page. If there were only 10 settings there, it would seem easier.

About the default style, the option is also duplicated, you can set the style from Board Settings, and also from Customise>Style>Details.

About the default style, it doesnt change for all users, only for guests and new users, because unless override user style is selected, the system selects the current default style as selected style for the user, but when you change the default style, it doesnt change for the user. So i think that when the user has 0 as style, he should have the current default style, not set the current default style. I did something related to this... and maybe it should be closed https://github.com/phpbb/phpbb/pull/4930

Whatever you end up doing please bear in mind that almost any change (to either core or style) will almost certainly end up "breaking" extensions and that will be a sure fire way to upset both extension developers and board Admins who use/rely on them.

DavidRemember: You only know what you know -
and you do not know what you do not know!

Allow subscribing to topics/forums - (I can't see a reason why you should disallow this)

Allow bookmarking topics - (Either remove the option, because why would you want to disable it, or remove the function. It's 2017 - modern browsers can handle this too)

Allow birthdays - (Remark: The option for the birthday listing at the bottom of the index should not be removed, just the option to allow/disallow users to enter their birthdays since I don't see a reason why you should want to disallow this..)

Display subject of last added post on forum list - (They are automatically hidden if the user has no permission to read topics)

Load settings - (This area should go to the page Load settings)

Allow styles to display custom profile fields in memberlist - (Why do style authors even have this option and does anybody ever use this?)

Display custom profile fields in private messages/user profiles/topic pages - (Admins can this enable/disable per field in another option)

Allow bookmarking topics - Please don't remove this feature completely. It is heavily used on my board.
Allow birthdays - Should probably be moved to an extension. I disable this on almost every install for a customer I've done (they don't want it).
All the others I'm ½ and ½ on. There's good arguments for keeping them and good arguments for removing them.

The ACP is not unusable because the number of settings available, but because of their organisation (or lack there of). Eg. "Board settings" and "Board preferences" are pretty *beep* names as you don't really know what you would find there.

It's both. If you want to toggle a particular feature (say the "board disabled" toggle), not only do you have to know/guess which category it is filed under, but then you have to read through a listing of 20 settings on that page. If there were only 10 settings there, it would seem easier.

What I meant is I see no point in removing settings. It just doesn't make sense to me. Features could be removed from core and moved to extensions (and thus getting rid of some settings). However, the problem is not the quantity of available settings, it is the organization of them. phpBB ACP has a great UX/UI if you ask the support team first how to do what you want to do.

What I meant is I see no point in removing settings. It just doesn't make sense to me. Features could be removed from core and moved to extensions (and thus getting rid of some settings). However, the problem is not the quantity of available settings, it is the organization of them.

I mostly agree with you, removing settings for the sake of removing settings isn't the goal. But removing settings which are duplicated on multiple pages is a good start.

That doesn't mean there aren't some settings that could/should go away, because they were created in a 2002-2006 world where the whole Internet was different - shared hosts were less capable, user download bandwidth was lower, no mobile web, etc.

It does not matter whether it all happens at once or happens piece meal the problems will still be there. In some respects it would probably, from an extension developers perspective, be better if it all happened at the same time as that would mean one set of changes (albeit a large set of changes) in preference to constantly changing our extensions - which, incidentally, makes more work for the Extensions Team.

DavidRemember: You only know what you know -
and you do not know what you do not know!

well also if it all happened in at once and to include a re-design as well i doubt we would want to release it under the guise of a minor release which would be from your perspective a non-issue in that case.